Update faq.md (lynndylanhurley#1401)

Updated FAQ to add **allow_password_change** column if not already added in existing User model.

docs/faq.md

@@ -124,7 +124,10 @@ You may be interested in [solidus_devise_token_auth](https://github.com/skycocke
     add_column :users, :tokens, :text
 
     # if your existing User model does not have an existing **encrypted_password** column uncomment below line.
-    # add_column :users, :encrypted_password, :null => false, :default => ""
+    # add_column :users, :encrypted_password, null: false, default: ''
+
+    # if your existing User model does not have an existing **allow_password_change** column uncomment below line.
+    # add_column :users, :allow_password_change, :boolean, default: false
 
     # the following will update your models so that when you run your migration
 
@@ -145,6 +148,7 @@ You may be interested in [solidus_devise_token_auth](https://github.com/skycocke
 
   def down
     # if you added **encrypted_password** above, add here to successfully rollback
+    # if you added **allow_password_change** above, add here to successfully rollback
     remove_columns :users, :provider, :uid, :tokens
   end